1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to an image forming apparatus with an improved construction so that the conveying velocity of a print medium can remain relatively constant.
2. Description of the Related Art
In general, image forming apparatuses are classified into monochromatic image forming apparatuses and color image forming apparatuses. A monochromatic image forming apparatus is an apparatus for forming a black-and-white image using only one color developer, and a color image forming apparatus is an apparatus for forming a color image using colors such as magenta, cyan, yellow and black.
Typically, in an electrophotographic image forming apparatus, a laser beam is scanned by an exposure unit and a latent image is formed on an image carrier electrified to a predetermined potential by an charging unit. The latent image is developed with developers and then transferred to print paper as a visible image while the print paper is fed and conveyed. A conventional color image forming apparatus overlaps transfer respective color-separated images, which are respectively developed on color-separated image carriers, directly onto a print medium. In this case, the print medium passes through plurality color carriers in sequence while being conveyed by a conveying member such as a PTB (Paper Transfer Belt), thereby receiving the overlapped color images.
In addition, the print medium having the overlapped color images pass through a series of image-fixing steps. Then, the print medium is discharged out of the image forming apparatus.
In order to transfer overlapped images directly onto a print medium as described above, it is relatively important to stably convey the print medium. Accordingly, the print medium is charged by the charging unit before it is fed by a paper-feeding roller from a paper feeding device and enters into a firstly located image carrier. The charged print medium moves into close contact with a conveying belt due to static electricity. Therefore, the print medium passes through plurality image carriers in sequence while being conveyed with the conveying belt in close contact with the conveying belt. Therefore, color images are formed on the image carriers and are overlapped onto the print medium. Here, image transfer rollers are mounted on predetermined areas, each corresponding to the image carriers, to provide a predetermined level of transfer pressure for compressing the conveying belt against the image carriers. Thus, the images on the image carriers can be efficiently transferred to the print medium.
With the conventional color image forming apparatus as described above, a print medium fed toward image carriers is subjected to high forces in the conveying direction of the print medium by the transfer rollers. The print medium is charged from the leading edge thereof by the charging unit at the initial entering stage and then the print medium is conveyed in close contact with the conveying belt at the leading edge area thereof. At this time, the paper conveying velocity and the force provided by the conveying belt may be different from those provided by the transfer rollers. Therefore, at the initial stage, where the leading edge area of the print medium starts to come into close contact with the conveying belt, the print medium starts to receive transferred images. The color images may not be correctly transferred to corresponding positions. As a result, color registration may be affected. In particular, because the color image formed on the firstly located color carrier has a large phase as compared to those formed on the other image carriers, the quality of a full-color image may deteriorate.
Accordingly, there is a need for an improved image forming apparatus which conveys a print medium at a relatively constant velocity.